1. Field of the Invention
The present invention relates to a self-healing system, primarily for repairing a break in an insulation material, including a plurality of microcapsules containing at least two reactants that form a polymer upon the rupturing of the microcapsules.
2. Description of Related Art
An electrical conductor generally contains electrical wires that are protected by surrounding the electrical conductor with an insulation material. Due to various stresses applied to the electrical wires and insulation material, a break may occur in a portion of the insulation material. Often, this break is not observed or even monitored. Additionally, any such break in the insulation material may, because of inaccessibility, be difficult to repair. Insulation breaks can cause the electrical wires to short, thus acting as a source of ignition if combustibles are present. Additionally, the breaks may lead to the prevention of power transmission, the monitoring of a transducer or the control of a relay valve. This, in turn, may lead directly to a catastrophic breakdown of an electrical system. Typically, a break in the insulation material may go undetected for an extended period of time before an electrical problem occurs, which may endanger the entire electrical system. For example, catastrophic failures could occur if the electrical system is present in aircraft and spacecraft, such as the NASA space shuttle.
Conventional methods of repairing the insulation material result in a repair that has a much larger diameter than the original insulation material and the thermal properties of the repaired insulation material are diminished. For example, Boeing procedure OEL (orbiter electrical)-4000—Wire/Cable: Mystik Tape Repair for 0-10 AWG Single Application requires that twelve layers of Mystik 7503 (Teflon tape, ½ inch wide, pressure sensitive adhesive) are wrapped over the break in the insulation material. Then the end of the Mystik 7503 tape wrap is secured with a spot tie. The type of insulation material repaired by this procedure is not specified, but regardless, the pressure sensitive adhesive will creep under a constant load at elevated temperatures. In another example, Procedure OEL-4020—Wire/Cable: Clamshell Repair of Primary Insulation, the area around the break in the insulation material is abraded with 320-grit sandpaper. A sealing sleeve, which has an inner sealing and an outer insulating sleeve, is cut lengthwise and the two sleeves are separated. The sleeves are placed over the break in the insulation material and the slits are aligned 180 degrees apart. The sealing sleeve is clamped tightly with a tweezer-clamp. A heat gun is used to heat the tweezer clamp until the sealing sleeve oozes out both ends of the clamp. The procedure cautions that great care must be taken to prevent damage to surrounding wiring or other objects. In this case, the heat gun used to melt the sealing sleeve risks damage to surrounding materials. In the examples given above either the strength of the insulation material after repair is greatly reduced or there is risk that heat damage may occur to surrounding materials due to the heat gun.
The primary insulation material used in the NASA Space Shuttle and aircraft are polyimides, preferably KAPTON, and polyfluorocarbons, preferably TEFLON. Both of these materials are chemically inert, have high working temperatures, and good electrical insulating properties. KAPTON, developed by DuPont, is the primary insulation material used in commercial and military aircraft. There is a series of KAPTON polyimide polymers that have the general chemical structure that is given below in Formula 1:
Polyimides can be prepared by reacting dianhydrides with diamines to yield poly(amic acids). Once the poly(amic acids) are heated, a rearrangement occurs followed by a loss of water to produce the polyimides. This chemistry was successfully commercialized by DuPont under the trade name of KAPTON. There are a number of KAPTON products produced by DuPont that have a range of physical properties. These properties range from materials that have no melting points, i.e. they decompose before they melt, to copolymers that are heat-sealable. There are many examples published in the chemical literature that describe methods of preparation of polyimides. The materials can be prepared in a two-step process as described above or a single step process can prepare them.
A large number of polyfluorocarbons, such as TEFLON, are known and the methods of synthesis have been well documented in the literature. Most of the preparation procedures for fluorocarbon polymers start with gas-phase reactions at high pressures. Preparation of these materials would not be practical on a small-scale, which means that it is unlikely that a direct synthesis method could be found for the polyfluorocarbons. However, heat-sealable materials are commercially available, as indicated in the discussion above under the clamshell repair method.
Many chemical reactions are exothermic, i.e., combustion processes, which rely on oxygen in the air to react with a fuel. Other materials release heat when two reagents are combined, as illustrated by the hypergolic reactions that occur when hydrazine is reacted with nitrogen tetroxide. These reactions can occur in the gas, liquid, or solid phases and their rates cover a wide range. Some compounds react under very controlled conditions to produce products that are non-hazardous and/or non-toxic. For example, development of the chemical heater for the United States Army meals-ready-to-eat (MRE) led to a number of controlled reactions that may only require water to initiate the exothermic reaction. For repair systems that require local heating to melt a specific material or to stimulate a polymerization or rearrangement reaction, chemical heaters could be the solution. This approach would apply the heat energy to the specific location and minimize the impact to other components.
While there is no known self-repairing electrical insulation material, there has been some recent investigation with composite materials. In particular, it is known to encapsulate a reactive monomer and disperse a polymerization catalyst in the structural composite. The arrangement of dispersing the catalyst in the structural composite requires that the reactive monomer diffuse through the structural composite before a repair can be initiated. Therefore, it would be advantageous to devise a self-healing system for insulation material that works immediately after the break in the insulation material occurs.